1 Introduction

Possible effects of truck platooning, especially on propulsion systems, are being discussed in this chapter. Improving vehicle performance is one of the key issues for reducing the environmental footprint of long-distance freight transport. Measures oriented to reduce emissions of \(\text {CO}_2\) and pollutants can be classified in two blocks, namely propulsion system and vehicle design.

Propulsion System: Beside the optimisation and adaptation of ICE-driven powertrains for renewable fuels, another trend is the increasing rate of electrified vehicles like battery electric vehicles (BEVs), hybridised electric vehicles (HEVs) or plug-in hybridised electric vehicles (PHEVs), including the electric energy storage not only using batteries, but also from fuel cell or from high-efficient ICE energy converters running on hydrogen, as well as electric road systems.

Vehicle Design: The main parameters are the aerodynamics and the rolling resistance. The optimisation of the aerodynamics is today restricted by regulatory constraints, which are likely to be relaxed soon (e.g. platooning). The topic aerodynamic advantages due to platooning is discussed in detail in Chaps. 5 and 6. Combining automated driving and electrified propulsion systems can substantially improve the road freight transport efficiency, although high costs of the battery electric heavy-duty vehicles (HDVs) are hindering their adoption by transportation companies.

2 CO\(_{2}\,\)Emission Reduction by Different Application Domains

Footnote 1Compared to 1990, domestic greenhouse gas emissions in the EU are to be reduced by at least 40% until 2030. All sectors have to make their contribution. The road transport sector is particularly essential for reducing greenhouse gas emissions and decarbonising the economy in the EU [1]. Road freight transport is essential for the development of trade on the European continent. Trucks carry around 70% of freight transported over land. Emissions from HDVs (trucks, buses and coaches) account for about 6% of total EU \(\text {CO}_2\) emissions and 25% of road \(\text {CO}_2\) emissions in the EU. Without any further action, \(\text {CO}_2\) emissions caused by HDVs are projected to grow by 9% over the period 2010 to 2030 due to the increasing transportation activities. Since 2018, there has been a legislative proposal for \(\text {CO}_2\) emission standards for HDVs in the EU. In order to reach the proposed objectives for the new vehicles coming into the market, truck OEMs should most likely manage a fuel consumption decrease by more than 3% per year until 2030.

According to EUROSTAT 2018, about 80% of all freight transport (in terms of tonne kilometre) is realised on long haul (over a distance of 150 km or more).

The needs are obviously different by vehicle usage and application domain. High technologies will gradually be extended to different use cases, from simple to increasingly complex environments: the European Road Transport Research Advisory Council (ERTRAC) has defined various areas of application for trucks, classified from simple environment to very complex (from the challenge for an automated operation point of view) as follows [2]:

  • “Confined area:” Private area, terminals, ports \(\Rightarrow \) simple environment.

  • “Hub-To-Hub:” Partly public road, from companies to ports or terminals \(\Rightarrow \) relatively simple environment.

  • “Highways:” Public roads and highways \(\Rightarrow \) complex environment.

  • “Urban environment:” Cities and public roads \(\Rightarrow \) very complex environment.

As also stated in [2] the highway application is mainly responsible for the major part of the \(\text {CO}_2\) emissions. The urban environment or confined areas have less significant impact on the share of \(\text {CO}_2\) emissions; therefore, we focus the following discussion on the area of highways.

3 Ultra-low Emissions on Highways and Zero Emissions in Cities

Footnote 2ERTRAC stated in [2] that potential and applicability of alternative propulsion technologies and fuels varies depending on different factors and the characteristic driving profile of a HDV. By 2030 and beyond, BEVs, requiring local charging stations, will be more suitable for short and medium distances. For long distances, hybridised powertrains with ICEs running on sustainable low-emission liquid or gaseous fuels, or alternatively electrified fuel cells, will be more suitable.

In February 2020, IVECO and FPT Industrial announced the plan to form a joint venture to equip Class 8 heavy-duty trucks through fuel cell technology.

The electricity sector is not yet fully decarbonised, though this is of course the goal in a far-future scenario. Energy storage, sourcing of battery materials and grid balancing might be the biggest practical issues at the time of increasing demand for electricity. Hydrogen is an energy carrier. Despite hydrogen being largely present in nature, it is not available as a pure element, so it must be produced using other sources of energy. The life cycle GHG emissions of the whole value chain (feedstock and energy) has to be considered.

4 Get the Right Infrastructure for Vehicle Energy Supply

A key ingredient of decarbonisation of future transport systems will be the availability of electrified vehicles including hybridised powertrains with ultra-low emission ICEs powered with low-carbon fuels. The development of both depends upon the rapid growth of an energy distribution network. This is relatively easy for liquid biofuels and liquid e-fuels, as it requires minor adaptation of the existing one. It is more challenging for gaseous low-carbon fuels (biogas, \(\text {H}_2\)), due to the cost of the refill stations. And, it is also challenging for electricity, due to its strong impact on the electricity grid and power stations that need to be implemented in size (with appropriate design guidelines and policy for making charging stations accessible for trucks) and electric power, to cover fast-charging energy needs.

5 Different Topologies for Truck Drives

The transition to a more sustainable freight transportation sector requires the widespread adoption of electric vehicles powered by batteries (BEVs) or fuel cells (FCEVs) beside the possibility to hybridise powertrains with plug-in functionality (PHEVs).

Figure 15.1 shows schematically different propulsion topologies—relevant for platoon able trucks—explaining how different kind of power sources are linked to the drive train. (a) shows a typically PHEV in a parallel application with plug-in functionality, (b) shows a pure BEV (battery-driven electric vehicle) (c) a serial hybrid with ICE as range extender and (d) a fuel cell EV.

In (a), the engine as well as the electric machine is each connected to the front and rear axle. The parallel configuration shown in (a) is characterized by a higher transmission efficiency due to the mechanical link between ICE and the electric motor.

An advantage of a series hybrid—shown in (c)—is that the engine operates at its maximum efficiency point(s) thanks to the buffering of excess power; however, one disadvantage is the relatively low transmission efficiency at relatively high vehicle loads compared to other hybrid configurations. If the vehicle does not mainly drive in urban traffic outside the urban area, relatively large electric machines (kW) are needed to provide power for high vehicle speeds. In (d), the ICE is replaced through a fuel cell, used as a primary power source. Topologies (c) and (d) require less battery capacity than a pure BEV, visualised in topology (b).

Fig. 15.1
figure 1

Truck propulsion topologies, from left to right: a PHEV, b BEV, c EV + REX, d FCEV; B...battery, E...E.-motor, F...fuel tank, H\(_2\)...hydrogen tank, G...generator, I...internal combustion engine, L...power-split, T...Transmission, FC...Fuel cell

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5.1 Truck Propulsion Systems for Highway Domain

Internal combustion engine propulsion systems are expected to remain the short- and mid-term technology of choice for long-distance intercity freight transportation. This will require continued focus on low emissions and very high energy conversion efficiency. Possible measures as renewable fuels, waste heat recovery and hybridisation should be seen in a short/mid-term time range, electric battery/fuel cells systems are the key technologies for the long-term range. Regarding vehicle energy density, for ICE-electric or fuel cell electric solutions, hydrogen offers a better alternative than electric batteries. But production efficiency of hydrogen and re-fuelling/charging infrastructure development need is also to be addressed, see Fig. 15.2.

Fig. 15.2
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Roadmap of possible technologies for highway domain

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5.2 Truck Propulsion Systems for Urban Domain

In future, short and medium distances may be more fitted for electric batteries or alternative powertrains to reduce \(\text {NO}_x\), particle and noise emissions. Fully electric vehicles, improvements in battery technology, reduction of cost, mass and improvement life cycle impact are essential for the market up-take of electrified HDV for urban use.

For the different application domains, possible propulsion systems are discussed as an example for highway and urban domain, see Fig. 15.3.

Fig. 15.3
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Roadmap of possible technologies for urban domain

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6 Importance of Thermal Management Concepts for Truck Drives

6.1 Motivation

HDV manufacturers constantly work on improving the fuel efficiency of the vehicle. This may be achieved on the engine side by a more efficient combustion process or at the chassis by increasing the aerodynamic parameters. Significant gains in the order of 5–10% can also be achieved by platooning, thus when one HDV travels in the slipstream of another HDV. However, the engine of an HDV needs to be cooled by the airflow through the engine compartment. This air mass flow through the radiator is significantly reduced while driving in a platoon for the following trucks. This may cause an insufficient cooling of the engine and may result in an engine failure [3]. To investigate the thermal behaviour of a platoon’s vehicle, Connecting Austria project partners made tests on a closed testing area. The second HDV of a platoon consisting of three vehicles was equipped with sensors to measure coolant inlet and outlet temperatures, heat exchange and pressure loss of the cooling circuit.

6.2 Materials and Methods

The automotive proving ground in Zalazone, Hungary, [4], provides ample possibilities for dynamic on road tests of vehicles. It consists of numerous test tracks designed for a wide variety of purposes. Similar to the first measurement campaign, the second tests were performed at ZalaZone’s handling course as well. This campaign incorporates additional tests related to the thermal behaviour of the cooling system of a truck in a platoon formation. The truck configuration consisted of two HDV (Volvo FH 540). An engine load trailer was additionally attached to the second HDV to increase/vary the engine load depending on the test. The platoon was equipped with different sensors which are depicted in Fig. 15.4. For more details, see Sect. 6.2.3.

Fig. 15.4
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Platoon sensors and load trailer for thermodynamic tests conducted at ZalaZone proving grounds

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As described in Sect. 6.2.3, the monitoring of the cooling system of the truck was realised by a coolant volume flow sensor, a coolant differential pressure sensor as well as coolant temperature sensors at the inlet and outlet of the radiator. Also, the speed of the coolant air was measured at four different positions. In addition, the fan speed was recorded.

A laser distance sensor was installed at the front of the second HDV. Wind speed and wind direction, ambient air temperature and barometric pressure were measured at the weather station located some hundreds metres nearby the test section. Data was collected by highly synchronised data acquisition systems and transmitted via WLAN to a master data acquisition system. To increase the engine load, a trailer dynamometer (manufacturer: Unsinn) was used. The braking power was set‘ to 350 kW.

The distance between the two HDV was set to 7, 11 and 22 m. Each measurement consisted of monitoring the thermal parameters for several laps of the test circuit. The vehicle speed was set to 80 km/h.

6.3 Results

The coolant inlet temperature is the most important parameter for the assessment of the cooling circuit. The mean inlet temperature for test scenarios of a vehicle distance of 7 m (inset a.), 11 m (inset b.) and 22 m (inset c.) is shown in Fig. 15.5. For a vehicle distance of 11 m and 22 m, the inlet temperature was around 50\(^{\circ }\)C at the start, increased to 92 \(^{\circ }\)C at the second lap and remained stable at this temperature level for the remaining laps.

Fig. 15.5
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Inlet temperature of the coolant for different vehicle distances

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However, the coolant inlet temperature at a vehicle distance of 7 m showed a different behaviour. The starting temperature was 60 \(^{\circ }\)C, increased to 91 \(^{\circ }\)C at the fourth lap but never converged to a stable temperature. There is still an increase of the temperature from the fifth to the sixth lap. Thus, there is an indication that the cooling system fails at very close platooning under a heavy load. In case of an insufficient heat exchange, the radiator fan has to increase the air mass flow. The results are shown in Fig. 15.6. The rotational speed of the fan increased to the maximum of 6000 rpm at the second lap for vehicle distances of 11 and 22 m and remains at this level for the remaining laps.

Fig. 15.6
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Fan speed for different vehicle distances

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6.4 Discussion

Platooning leads to a reduced fuel consumption as the drag coefficient is significantly reduced in the slipstream. However, this also leads to a reduced radiator air mass flow, a reduced heat exchange and may result in a malfunctioning of the engine’s cooling circuit. Measurement at the proving ground in ZalaZone indicated that for a vehicle distance of 7 m under a friction load of 350 kW the radiator inlet temperature does not converge. For larger vehicle distances a stable coolant radiator inlet temperature was reached. For all three distances, the radiator fan was operated at maximum speed. The operation of the fan may consume a part of the fuel consumption gain. Additionally, it has to be investigated if the radiator fan is designed for long operation under maximum speed.

7 Cooling Concepts on the Example of H\(_{2}\) Driven Trucks

Due to the lack of air flow at the front heat exchanger with platooning, higher effort must be put in cooling the propulsion system. Especially when looking towards combustion engine and fuel cell engine, improved cooling performance needs to be ensured. In case of a pure electric propulsion system, heat fluxes for dissipation to the surrounding are much lower and less challenging. As nowadays a pure electric-operated HDV truck is not economic due to high battery weight, a serial hybrid concept is most promising. This comprises of an electric motor mechanically connected to the wheel’s axes. The battery can be laid out, for example 100km pure electric driving (inner city). The major energy is provided by the range extender, either an ICE, operated in its sweet spot or by a fuel cell. With respect to emissions, the fuel cell is seen as the favourable solution for the near future.

In Fig. 15.7, the Sankey diagram for heat flow of a fuel cell is shown. Approximately, 33% of the \(\text {H}_2\) energy are dissipated to the ambient, while due to low temperature difference, a high constant mass flow must be ensured. Therefore, a bigger heat exchanger area and additional fan power is required. As platooning is contrary to this requirements, additional effort must be made.

Fig. 15.7
figure 7

Sankey diagram for heat flow of a fuel cell

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The concept in Fig. 15.8 is to be seen as the most common approach. This system is also called a two-circuit system as there is one low-temperature (LT) cycle for the e-motor and the power electronics, whereat especially the latter is in need of lower coolant temperatures which does not allow any satisfying use of one overall circuit architecture. The second circuit serves for cooling the stack and is called high-temperature (HT) circuit. The system is driven by an electrically controlled pump, and a particle filter can be used to avoid fouling of the PEMFC and the HX. Then, the temperature control of the humidifiers is common with the release of heat prior absorbed form the FC. In a next step, the compressor, respectively, the intercooler (if existing), for the reactant air is conditioned. Via convection at the radiator, the heat is released to the ambient air. Subsequently, an ion exchanger is used in case of no use of glycol within the cooling circuit. This assembly ensures to reduce, i.e. soda-ions, and restock the water with potassium ions. For this reason, the water loses most of its electrical conductibility.

Fig. 15.8
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Two-circuit cooling system including low-temperature (LT) and high-temperature (HT) circuits

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The concept in Fig. 15.9 shows an approach for increasing the cooling efficiency of the HT-cooling circuit of the FC. By the use of an additional heat exchanger between HT- and LT circuit, respectively, HT- and A/C-circuit extra cooling performance can be gained. On the one hand, this goes along with higher (and for FC very important) cooling performances, and on the other hand, this leads to an increase of the total system efficiency because of better utilisation of the proper cooling capacities. It is required to execute vast prior investigations towards control and operation strategies in order to have a resilient validation for the temperatures in the cooling/conditioning cycle. The heat exchanger should be mounted in an exposed location (in front of the wheels), because of higher air flows at this position.

Fig. 15.9
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Two-circuit cooling system with an additional heat exchanger connecting the HT and LT circuit, increasing the cooling efficiency of the HT-cooling circuit for the FC

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8 Outlook

To ensure sufficient cooling performance, active cooling with a refrigerant system as discussed above should be used. Therewith, the thermal drawback of platooning can be fully eliminated. As shown above, this approach requires a complex and big refrigerant system. In terms of future refrigerants with a low global warming potential, also propane can be investigated. Due to a current safety limit of 150g propane for refrigerant circuits, a compact refrigerant unit (CRU) should be considered. Such a system can significantly help to improve thermal efficiency of the powertrain without a big change of the vehicle architecture. Such a prototype was developed by virtual vehicle in the EU-project Optemus and is currently in research and development for series production, also for heavy-duty applications [5].

Another methodology is to implement a predictive thermal control. With the help of the predicted road profile in a virtual model, the future heat fluxes are estimated and the thermal system behaviour for the next horizon can be estimated. Therefore, countermeasures to reduce the temperature can be already started at a very early stage, and high temperature peaks are avoided. In general, also lower auxiliary’s energy consumption can be achieved with this approach due to operating at higher coolant temperatures, respectively, at better aggregates’ performance points, see [6]. With prediction, the optimal time for driving in the platoon is estimated and the platoon can be selectively dissolved if too high thermal loads occur. In the case of upcoming hill climbs or city parts, for example, a dissolution much in advance to reaching temperature thresholds is started. In turn, at “normal” highway drives, platooning can be kept up long and potentially even at higher threshold temperatures as safety margins can be reduced.

Beside the described predictive thermal management, a predictive energy management (with focus on the operation of powertrain components) needs to be pursued. It allows HDV applications in platooning to increase the efficiency of the power source system (fuel cell and battery). Further, this serves to mitigate the degradation of the components and to increase its lifetime. Also, when looking towards the state of charge of the battery, predictive measures will ensure to sustain battery charge as far as possible during platooning. With the focus on fuel cell electric vehicles (FCEV), future research focus for cooling is needed, especially due to the following facts [6, 7]:

  • Reduction of large radiators cooling surfaces.

  • Simplification of thermal management and reduction of parallel circuits with different temperature levels.

  • Focus for predictive strategies towards uphill and platoon sections that have significant heat losses.

  • The degradation of fuel cell modules is increased by frequent starts and shutdowns. Therefore, predictive strategies for activation and shutdown of fuel cell modules are required.

Due to the growing demand of transportation of goods, implementing only one of these measures discussed will not be sufficient enough to significantly increase fuel consumption efficiency and therefore reducing the environmental impact [8]. Therefore, the combination of automated driving and electrified propulsion systems will substantially improve the environmental impact and road freight transport efficiency. More details can be found in [9] in section “Advanced Vehicle Concepts 2020+”, subsection “Research Requirements for Digitalization and Automatization of Vehicles and Infrastructure”.